Advances in mobile communications have opened up enormous opportunities in cellular communications, terrestrial PCS communications, wireless local loop and satellite PCS communications through medium/low earth orbiting satellites. With the increase in complexity of services and density of the electromagnetic radiation environment, spectrum is becoming increasingly scarce. The effects of fading caused by absorption in the transmission medium, and changes in the propagation characteristics of waves due to multiple reflections, diffraction and scattering from objects in the medium such as buildings, terrain variations and moving vehicles, have been investigated at frequencies below 1 GHz and also at frequencies in the 2.0 GHz band. Current interest has centered on determining propagation characteristics at 28 GHz/40 GHz, which are expected to be of great interest in the future for PCS communication through very low earth orbiting satellites.
Many techniques have been developed to minimize these effects, one of the most effective being diversity reception. The implementation of a diversity reception scheme depends on the arrival of two independently fading signals which have LAW OFFICES comparable signal levels. The cross-correlation between the envelopes of these incoming signals is used as an indication of the independence, and the amount of cross-correlation, known as cross-correlation factor, determines the degree to which the rate and depth of fading may be reduced. The cross-correlation factors should be as low as possible to achieve maximum benefit from diversity reception and consequently achieve diversity gain. In practice, designs are aimed at getting a cross correlation factor not exceeding 0.7.
In diversity reception of the type known as space diversity reception, reception occurs via two spatially separate paths. Recent investigations on the effects of space diversity reception at a base station antenna in the 1800 GHz band have shown that, if the base station antenna is inclined with reference to the mobile antenna, there is an appreciable degradation in the received signal level. This degradation has been found to be of varying extent for different angles of inclination with reference to the mobile antenna.
Unfortunately, transmission from mobile antennas (uplinks) undergo random changes in polarization due to diffraction and scattering by objects, and furthermore the orientation of mobile antenna is also random depending on how the mobile user holds the handset. As a result, the plane of polarization of linearly polarized waves in the uplinks undergo random changes, resulting in fading of the signal received at the base station antenna.
Another problem caused by reliance on space diversity is that quite often the mounting of two antennas on buildings and towers with the required horizontal and vertical spacing for space diversity pose engineering and zoning concerns--particularly in systems which employ small cells and antennas at heights lower than the surroundings.
If the base station antenna is invested with the capability to discriminate the incoming waves in orthogonally polarized planes, and if this discrimination can be quantitatively measured in dB as cross polarization discrimination factor, experimental work has indicated that this figure may be in the order of 6 to 20 dB.
This leads to the possibility of taking advantage of "polarization diversity" at base station antennas.